A virtual machine (VM) is a software abstraction, or “virtualization,” of an actual physical computer system. Each VM typically mimics the general structure of a physical computer and as such will usually have both virtual system hardware and guest system software. The virtual system hardware typically includes at least one virtual CPU, virtual memory, at least one storage device such as a virtual disk, and one or more virtual devices. All of the virtual hardware components of the VM can be implemented in software to emulate corresponding physical components. The guest system software typically includes a guest operating system and drivers as needed.
Frequently, rather than starting up a new virtual machine from scratch by allocating resources for it, loading and booting an operating system, and then loading and starting specific applications, system operators find it useful to create new working virtual machines that are copies of existing virtual machines, including machines that are currently running and those that are powered off. To start up a virtual machine in this way, a typical process starts with the creation of a new virtual machine on a suitable host that is a copy of an existing source virtual machine. To clone the contents of an existing source virtual machine that is in a running state, a “snapshot” of the source machine file system is taken, typically with the aid of software residing on the source machine. The snapshot provides an image of the complete state of the source machine file system at a moment in time. To clone a virtual machine that is powered off, a snapshot is not required.
In order for a new VM to be a copy of a source machine, it is cloned to storage associated with new virtual machine. After cloning has been completed, the operating system of the new VM may be loaded and booted, and specific applications of the new VM may be loaded and started. There are many ways of performing this cloning operation. In a typical computing environment, the new VM is connected to the source machine over a network, and the data are transferred over that network. The VM storage can be local (such as all or a portion of a disk drive attached directly to the physical machine which hosts the VM), or it can be located remotely, for example, in a Storage Area Network (SAN) accessible from the VM. Regardless of the specific hardware and software details, the cloning operation can take a considerable amount of time. The amount of time will vary according to the implementation details, the resulting average data transfer rates, and the size of the source machine file system that must be transferred. For typical machines and networks in common use in 2008, the size of the source file system can be 50 GB or more, and typical average data transfer rates can be about 20 MB/s. Thus, a complete cloning operation can take at least 2500 seconds or about 40 minutes. Significantly longer times are not uncommon, especially for servers running data-intensive applications that can have much larger file systems.